A standard motor-vehicle door latch is mounted on a door edge and has a latching fork that can engage around and retain a bolt projecting from a doorpost. A pawl can move between a retaining position engaging the fork and holding it in a position retaining the door bolt and a freeing position allowing the fork to pivot and free the door bolt. This pawl in turn is actuated by at least one actuating lever normally in turn operable by an outside door handle and, in most situations, by an inside door handle.
Typically the actuating lever pivots about an axis that extends horizontally in the vehicle travel direction. Thus in a side-impact collision, the considerable inertia of the actuating lever can cause it to pivot, actuating the pawl and releasing the bolt even if the handle it is connected to does not move, as lost-motion couplings are normally provided to allow the inside and outside handles and actuating elements to work independently. Thus when hit on the side the latch releases and the door can spring open, with the obvious safety problems.
Accordingly British patent 1,413,887 describes a latch having a blocking lever which is connected via a pin directly to the pawl. An extension of the blocking lever engages into a recess of the latch housing so that when the pawl is actuated by means of the actuating lever the blocking lever is similarly raised and then lowered. It is possible in a collision for this blocking lever to fail to block operation of the pawl, in particular if the latch is rusted or dirty. Furthermore the blocking lever can cause the pawl to bend, ruining the latch.
In another system described in German patent 1,678,024 an L-shaped blocking lever is pivoted on the lock housing and can swing, when moved inertially as in a crash, into a position blocking movement of the actuating lever. During normal operation, however, the blocking lever never moves so that it is possible that after a long time it becomes stuck in place, by rust and/or dirt, so that it does not work.
German patent document 195, 11 651 of Hubner shows another system with a hook-type blocking lever pivoted on the housing and engageable with the actuating lever to arrest it in an accident. This arrangement is fairly complex and adds substantial cost to the latch. The same is true for the system shown in German patent 4,117,110 of Claar.